1. Field of the Invention
This invention relates to a digital signal reproducing circuit for reproducing digital signals, and particularly to a digital signal reproducing circuit for reproducing signals while detecting the phase difference and jitter components of the signals to be reproduced.
2. Description of the Related Art
A digital signal reproducing circuit is adapted for reproducing digital data from an input analog signal, and has a synchronous clock reproducing section for generating a clock synchronous with the input analog signal, that is, a PLL circuit.
The synchronous clock reproducing section performs such control as to generate a clock substantially equal to the clock frequency of the input signal and generate a phase difference signal with a pulse width detected from the phase difference between the generated clock and the incoming signal, thus synchronizing the generated clock with the clock component of the incoming signal on the basis of the phase difference signal.
In a disc reproducing device for reproducing signals recorded on, for example, an optical disc, a digital signal reproducing circuit 202 is included, as shown in FIG. 1.
The disc reproducing device 201 includes an optical pickup 214 for radiating a light onto an optical disc 200 so as to read signals recorded on the optical disc 200 on the basis of a return light of the radiated light, a waveform equalizer 203 for performing waveform equalization of reproduction signals outputted from the optical pickup 214 so as to output the waveform-equalized signals, an asymmetry correction circuit 204 for performing asymmetry correction of the signals outputted from the waveform equalizer 203 so as to output the asymmetry-corrected signals, and a binarization circuit 205 for converting the signals outputted from the asymmetry correction circuit 204 into binary signals so as to output the binary signals. The disc reproducing device 201 also includes a phase comparator 207 for outputting phase difference signals obtained by comparing the binary signals outputted from the binarization circuit 205 and a synchronous clock, a loop filter 208 for averaging the phase difference signals outputted from the phase comparator 207 in an analog manner so as to output the averaged signal, and a voltage-controlled oscillator 209 for generating a synchronous clock in such a manner that the signal outputted from the loop filter 208 becomes zero (0). In addition, the disc reproducing device 201 includes a jitter measuring circuit 210 which is constituted by an absolute value circuit 211 and a low-pass filter 212 and adapted for measuring jitter components on the basis of the phase difference signals outputted from the phase comparator 207, a servo circuit 215 for operating an objective lens by a biaxial actuator in the optical pickup 214 so as to perform focusing servo and tracking servo, and a controller 213 for controlling the servo circuit 215 and the waveform equalizer 203.
The phase comparator 207, the loop filter 208 and the voltage-controlled oscillator 209 constitute the synchronous clock reproducing section 206.
In the disc reproducing device 201, jitter components of the signals inputted to the disk reproducing device 201 are measured on the basis of the phase difference signals outputted from the comparator 207 of the synchronous clock reproducing section 206, and offset adjustment of tracking servo and focusing servo is carried out on the basis of the measured jitter detection signals.
For example, the jitter measuring section 210 for measuring jitter components is adapted for calculating jitter detection signals by digital processing. Thus, the jitter measuring section 210 measures phase difference signals as input pulses by high-frequency sampling clocks and averages absolute values thereof so as to calculate jitter detection signals. The controller 213 performs offset adjustment of servo on the basis of the jitter detection signals calculated by the jitter measuring section 210.
Meanwhile, analog processing is inferior in offset and operational precision to digital processing, though absolute values of pulse width may be averaged relatively easily in analog processing. Also, it is difficult to calculate the square average. Therefore, when jitter components are measured by analog processing, the offset or operational precision becomes inferior.
However, in order to carry out digital processing, phase difference pulses must be sampled with a time resolution equivalent to the required precision. That is, a clock of a frequency which is the reciprocal of the time resolution, or a high-precision delay circuit is required.
Thus, in view of the foregoing status of the art, it is an object of the present invention to provide a digital signal reproducing circuit which enables measurement of phase difference and jitter components with high precision while realizing miniaturization of the circuit.